CE 320 Exam 3

The University of Alabama Environmental Engineering - Dr. Leigh Terry

How does organic pollution lead to fish kills?

The resultant oxygen depletion

What is a stream’s natural recovery process when dealing with oxygen-demanding pollution?

• Natural Reaeration

• Assimilation Capacity - As long as the pollutant load stays within the stream’s capacity, it can neutralize the waste and recover naturally

• Oxygen Balance

DO Sag

Dissolved oxygen concentrations decline as oxygen demanding pollutants are oxidized, then recover and the stream returns to a healthy condition

Effects of Temperature on Dissolved Oxygen

• Impacts the solubility of oxygen in water - lower temp water can hold more oxygen

• Impacts the rate at which microbes degrade oxygen demanding compounds

What are the worst-case conditions downstream?

The critical point (C)

Streeter-Phelps Model

A tool to assess ability of a stream to absorb an oxygen demanding pollutant model

When does deoxygenation occur?

Occurs as microbes degrade the waste introduced to the river and consume DO (aqueous O2)

When does reaeration occur?

Occurs as oxygen is resupplied to the river from the atmosphere

How does kd compare to k in shallow and fast-moving streams?

kd>>k

How does kd compare to k in deep and slow-moving rivers?

kd is similar to k

How do depth and turbulence affect the reaeration coefficient (kr) in a river?

kr is inversely proportional to depth and directly proportional to velocity

• High kr = shallow/fast

• Low kr = deep/slow

What is the DO Standard?

The minimum DO to protect wildlife

DO Standard vs DO at Critical Point

• DO @ critical point > DO Standard = OK

• DO @ critical point < DO Standard = remediation needed

Main factors that influence critical DO level

• Ultimate BOD (Lw)

• Qw

• DOw

• kd

• kr

Possible solutions for low DO

• Manage the waste stream

  • Lower wastewater volume

  • More effective wastewater treatment processes

  • Aerate wastewater effluent before discharging

• Extreme: aerate the river at the critical point

What are indicators of stream health?

DO and BOD

What does BOD model?

A 1st order decay reaction

Municipal Water Supply and Sanitation Process

Watershed Protection → Pumped → Drinking Water Treatment Plant → Piped Distribution System → Household → Sewers → Wastewater Treatment Plant → Discharged

Safe Drinking Water Act (SDWA)

• 1974

• Covers “public water systems”

• Gives the US EPA authority to set minimum drinking water quality standards

National Primary Drinking Water Regulations (NPDWR)

• Protects human health

  • Legally enforceable standards

National Secondary Drinking Water Regulations (NSDWR)

• Provide guidelines for aesthetic quality

• Not enforceable federally, just guidelines that states can opt to enforce

What is MCLs?

Maximum Contaminant Levels

Primary MCLs

To protect health

• Most common is an upper limit

• Some use a “Treatment Technique” (TT) requirement

What did the EPA pass in 2024?

First-ever national drinking water standard for PFAS

• Designated PFAS-PFOA and PFOS as hazardous substances under the Superfund

Secondary MCLs

To protect aesthetic quality

• Can have a maximum

• Can have a range

Public Water System (PWS)

Served by the SDWA

• 13-15% of Americans use private household wells for drinking water, which SDWA doesn’t apply to

• >150,000 PWS in the US

Community Water System (CWS)

Same customers year round, >250 million people served in the US

Non-Community Water System (NCWS)

• Non-Transient (NTNCWS) >/= 25 same people >/= 6mo/yr but not year round

  • 7 million people served

• Transient (TNCWS) camps, resorts, campgrounds

  • 13 million people served

Size Classification of PWS

Classified by number of people served

• Very small: 25-500

• Small: 501-3,000

• Medium: 3,001-10,000

• Large: 10,001-100,000

• Very Large: >100,000

What are the major technical goals of drinking water treatment?

• Remove particles (including sediment, microorganisms, etc.)

• Kill any remaining microorganisms that aren’t removed

Conventional Surface Water Treatment for Drinking Water

Screening (removes sticks, leaves, etc.) → Rapid Mix → Flocculation → Sedimentation (sludge exits) → Filtration → Disinfection → To distribution

What is the purpose of each unit process in drinking water treatment?

• The first 4 units work together to remove particles/dissolved contaminants from water

• Disinfection is a final step for killing any remaining pathogens

What does rapid mix do?

A chemical coagulant is added to the water and a high-energy mixers disperses the chemicals to neutralize the charges on the particles so they are ready to stick together. (Alum, Al2(SO4)3)

What does flocculation?

The water is gently mixed and forms “floc”, large, heavy clumps, of the particles. This allows the water to move onto the next step, sedimentation as the heavy floc can now sink to the bottom.

How does sedimentation work?

Allows time for large floc to settle

• Large alum complexes “sweep” through water capturing smaller molecules and particles, pulling them to the bottom

Inlet Zone

Purpose is to evenly distribute flow across sedimentation basin

Settling Zone

Enable design flow, slow enough to allow floc to settle out

Outlet Zone

Weirs provide large area for water to flow out: prevent high velocity from stirring up floc

What happens when the floc settles?

The solids are removed as wet “sludge”, dewatered settling pond or belt press), then shipped to land fill.

Filtration

Passage of water through a bed of porous media

• Goals: To remove remaining floc, particles, and chlorine-resistant pathogens

How does filtration work with disinfection?

It removes the most chlorine-resistant pathogens

How does disinfection work?

It kills the remaining pathogens, usually using chlorine

Why is chlorine allowed a “residual” in piped distribution systems?

• In case there’s subsequent infiltration of treated water

• To prevent overgrowth of biofilms in pipes

Redundancy

At least two of each unit process operated in parallel

ex. Need to backwash

What’s the most common coagulant?

Aluminum Sulfate (Alum)

• Traditional coagulants are salts of +3 cations, Al3+ or Fe3+

How do coagulants work? (chemical process)

Natural water is full of ions and natural organic matter (NOM) carrying charge, and these ions become concentrated around charged particles. So adding cations compresses the repulsive layers

What is “Jar Testing”?

Lab simulation of coagulation/flocculation

What are the two major design configurations of the sedimentation tank?

• Horizontal flow (rectangular)

• Up flow (circular)

What are the 4 zones of the sedimentation tank?

• Inlet Zone

• Settling Zone

• Outlet Zone

• Sludge Zone

Sludge Zone

Where solids are removed

Weir Loading Rate (WLR)

A standard design code parameter that allows the design of the outlet Weir Length (WL)

Assumptions of horizontal sedimentation basins

• Particles and velocity vectors are evenly distributed across the width and depth of the tank (what the “inlet zone” is built to accomplish)

• Liquid moves as in ideal slug down the length of the tank (plug flow)

• Any particle hitting the bottom of the tank is removed

What are filters in conventional DWT called?

Rapid sand filters

How do rapid sand filters work?

• Gravity driven based on elevation head

• Water → Sand → Support Gravel → Underdrain

How does backwashing work?

Pressure is used to rapidly drive water back up through filter → suspends sand and cleans filter → graded media bed

Why is it advantageous to use two types of filter media with different densities?

It prevents the top of the filter from clogging instantly, allowing the entire depth of the bed to pull its weight, leading to longer filter runs

What is commonly used with sand in the filters?

Anthracite coal